1. Field of the Invention
The present invention relates to magnetic recording media having a thin-metal-film recording layer, and particularly to thin-metal-film magnetic recording media for high-density magnetic recording which are suitable as video tapes for long-time recording and high-capacity tape streamers.
2. Description of the Related Art
Coating-type magnetic recording media are widely known for use as audio tapes and video tapes. These magnetic media are produced by applying a magnetic coating material to a nonmagnetic support and drying it. The coating material is prepared by dispersing a magnetic powder, such as a magnetic oxide powder or a magnetic alloy powder, into a binder, such as a vinyl chloride-vinyl acetate copolymer, a polyester resin, a urethane resin, or a polyurethane resin.
On the other hand, thin-metal-film magnetic recording media have recently been proposed with increasing demand for higher-density recording. These magnetic recording media are produced by forming a magnetic layer on a nonmagnetic support directly or with an extremely thin adhesive layer disposed therebetween by plating or vacuum film formation processes such as vacuum deposition, sputtering, and ion plating. The magnetic layer is formed of a magnetic metal material such as a CoNi-based alloy, a CoCr-based alloy, or CoO.
This type of magnetic recording medium has various advantages. For example, this magnetic recording medium has high coercivity and high remanence ratio, and exhibits excellent electromagnetic conversion characteristics at short wavelengths because the media includes a significantly thin magnetic layer. In addition, the magnetic recording medium exhibits significantly small recording demagnetization and extremely low thickness loss in playback. Furthermore, the magnetic layer of the magnetic recording medium contains ferromagnetic metal particles at high fill densities because a binder, which is a nonmagnetic material, is not contained in the magnetic layer, unlike coating-type magnetic recording media.
In addition, oblique deposition, that is, the deposition of a magnetic layer at an oblique angle, has been proposed and put to practical use to improve electromagnetic conversion characteristics and thus produce higher output.
For thin-metal-film magnetic recording media as described above, techniques such as the formation of a protective layer on a magnetic layer and the formation of a back layer on a main surface opposite a magnetic layer are commonly employed to improve, for example, durability and running properties.
Furthermore, thin-metal-film magnetic recording media with smoother surfaces have been produced to reduce spacing loss with increasing recording density. A smoother surface, however, has an increased contact area with a magnetic head, thus causing larger friction and shear stress on the magnetic layer. Accordingly, it is important to protect the magnetic layer from such severe sliding conditions by forming a protective layer thereon.
A back layer functions to reduce the dielectric constant of the surface of a nonmagnetic support, inhibit defective running due to charging, improve the durability of the nonmagnetic support, and protect the magnetic tape from being damaged by the friction of a running head or the tape itself.
One of the most frequently used materials for supports for thin-metal-film magnetic recording media is polyethylene terephthalate (PET) films; particularly, PET films with a thickness of about 7 to 10 μm are used for supports for home video cassette tapes such as 8 mm tapes, and PET films with a thickness of about 5 to 7 μm are used for tape streamers for data backup of computers.
In recent years, more compact magnetic recording media that enable longer-time recording for video cassettes have been demanded as more compact video cassettes are produced. On the other hand, tape streamers with higher capacities have been demanded with recent increases in the amount of information. Accordingly, thinner magnetic recording media have been studied to meet such demands. In particular, the capacities of tape streamers have been doubled every few years, and thus thinner magnetic recording media have been demanded to achieve higher capacities.
A simple approach to reducing the thickness of a magnetic recording medium is to reduce the thickness of a support; however, the strength of a support formed of a PET film decreases in the longitudinal or width direction with decreasing thickness. Decreased strength in the longitudinal direction causes the magnetic recording medium to readily deform during the running on a video cassette recorder or drive.
In addition, decreased strength in the width direction causes problems such as wrinkles, folds, and defective contacts with heads. To overcome the problems, polyethylene naphthalate films and polyamide films have been studied and put to practical use as supports for magnetic recording media.
Polyethylene naphthalate films and polyamide films feature high Young's modulus and high heat resistance compared to PET films. The high Young's modulus of the films allows a support for a magnetic recording medium to have a smaller thickness. These films have thus attracted attention as supports for magnetic recording media capable of providing video cassette tapes for longer-time recording and tape streamers with higher capacities.
Stiffness, however, is proportional to the cube of thickness; for example, an eightfold increase in the Young's modulus of the material for a support is required to halve the thickness of the support while maintaining the same stiffness. Accordingly, a simple reduction in the thickness of a support results in unsatisfactory mechanical strength.
Japanese Unexamined Patent Application Publication Nos. 56-16939 (Patent Document 1), 58-97131 (Patent Document 2), 57-78627 (Patent Document 3), and 57-37737 (Patent Document 4), for example, propose thin-metal-film magnetic recording media having a reinforcing layer disposed on the back surface of a nonmagnetic support to compensate for decreased stiffness as a result of a reduction in the thickness of the nonmagnetic support. According to the proposals, a back layer formed of a thin metal film can improve the Young's modulus, running properties, curl resistance, and mechanical strength, for example, of a medium.
The magnetic recording media according to the above publications, however, are not designed to achieve the optimum Young's modulus, and thus cause problems including decreased output and defective envelopes due to poor head touch and tape damage such as edge folding and single-side stretching. In addition, the magnetic recording media cause skew after storage, or cause cracks and shavings on magnetic layers, increased amounts of wear, and increased dropouts after running.
Japanese Examined Patent Application Publication No. 7-60503 (Patent Document 5), for example, has recently proposed an 8 mm video tape having a flexural rigidity EI controlled within a predetermined range. According to the publication, the video tape exhibits significantly high running stability, good head touch, little output decrease, and few defective envelopes.